This invention relates to analogs of enkephalin which display agonist activity at opiate receptors. Enkephalin, a naturally occurring pentapeptide, has been isolated and found to be a mixture of two pentapeptides which differ only in the amino acid present at the 5-position. Leucine.sup.5 -enkephalin is thus represented by the following structural formula ##STR1## and methionine.sup.5 -enkaphalin by the following formula ##STR2## wherein the Tyr, Met and Leu residues are all of the L-stereochemical configuration.